This invention relates to a focal point detecting device for a single-lens reflex camera wherein the image of an object is divided into two parts or halves by means of two wedge type prisms so that the two half-images are displaced from each other, and their relative displacement is varied according to the focusing conditions of the object.
A number of methods have been proposed in which focal point detection is carried out by utilizing variations in the contrast, spatial frequency and light quantity of an object when it is focused. Most of these conventional methods are not suitable for single-lens reflex cameras, however, because the detecting apparatus is sensitive to mechanical vibrations, too large in size, and too intricate in its construction.
In addition, it is very difficult to accomplish focal detection with a high degree of accuracy.
A method is also known in which the image of an object is divided into two parts or half-images by means of wedge type prisms so that the two half-images are displaced in opposite directions from each other, and focal point detection is effected from variations in the relative displacement of the two half-images. In this conventional method, however, it is necessary that the image of the object be sharp and clear even when the object is not in focus. Accordingly, in order to obtain the necessary image clarity, the device must be unduly large and is therefore not suitable for single-lens reflex cameras.